The Federal Communications Commission (FCC) has designated the frequency band between 1920 and 1930 MHz as an unlicensed band for isochronous personal communications services (UPCS). This unlicensed band will be used primarily to support wireless telephone and circuit-switched data. The maximum permitted transmission power of 55 mW for 300 kHz bandwidth limits UPCS to indoor use, because the power level is too low for effective use outdoors.
FIG. 1 is an example of a simple UPCS system 50. A customer has a mobile terminal (MT) 52, such as a wireless telephone or key set, which is in wireless communication with a radio port (RP) 54 connected to a radio port control unit (RPCU) 56. A UPCS system may have a number of RPs 54 connected to the same or a different RPCU 56. The RPCU 56 may be connected to a communications network, such as an ordinary telephone line 58 which connects to a local switch 60. The switch connects to a telephone network, such as a public switched telephone network (PSTN) 62. The customer may have personal information, such as a telephone or personal communications number, call forwarding and routing information, account information, credit and billing information, and the like stored in a telephone network database, known as the Home Location Register (HLR) 64, servicing the customer's home area.
The 1920-1930 MHz band was previously allocated to licensed point-to-point microwave communication. The FCC has instituted strict requirements on the use of UPCS systems, at least until all of the point-to-point microwave systems are cleared from the band. It is expected that this clearing process may take as long as seven years. To this end, the FCC requires that UPCS components cannot transmit if they move outside of certain authorized areas. For this reason, it is generally believed that only large, geographically bound systems, such as Centrex and PBX systems may be used until the band is cleared. It also implicitly requires that the RP 54, not the MT 52 (which is capable of moving outside of authorized areas), must initiate communications.
Because the UPCS band is unlicensed, it is available for use without paying the FCC a license fee for each piece of radio equipment installed. To permit the band to be used fairly by a number of radio ports, such as RPs from unlike systems or from a number of service providers, within the same area, the FCC requires an etiquette for obtaining channels in the UPCS band. This etiquette is called the Listen-Before-Talk (LBT) etiquette.
In the LBT etiquette, each RP must find an available channel in the band on which to transmit. The FCC has divided the band into eight 1.25 MHz channels. "Narrow band" (e.g., less than 625 kHz occupied signal bandwidth) communications devices are required to begin searching for available channels at the lower 3 MHz of the band. "Wide band" communications devices are required to begin searching for available channels at the higher 3 MHz of the band. The following description will refer to narrow band communications devices, but the reader understands that it is equally applicable to wide band communications devices. Protocols in the UPCS spectrum may divide each 1.25 MHz channel into a number of system channels.
The RP 54 begins its search by measuring a first channel (e.g., randomly selected from the system channels contained in the bottom 3 MHz of the band) in the band to determine if interference below a threshold (the FCC requires the threshold to be no larger than 30 dB above a background level, except in special cases) is present. Interference above the threshold level indicates that the channel may already be taken by a nearby RP and is not available. The RP monitors the channel for the threshold level of interference for 10 ms before it can transmit, to make sure that another RP has not already acquired the channel. If the channel has interference beyond the threshold at any time during the 10 ms period, it searches the next higher channel. An RP must acquire the first channel it monitors and detects no interference above the threshold for 10 ms.
Once the RP 54 has acquired a channel, it is required to transmit immediately on that channel to "advertise" its availability for communications with an MT 52. If the RP 54 does not establish communication with an MT within that 30 second time period, the RP must relinquish the channel, and begin again its search for an available channel.
PACS-UB is a protocol which is adapted for use in the UPCS spectrum. The following description uses the PACS-UB protocol to illustrate the present invention. The reader understands, however, the principles of the invention apply to any protocol operable in the UPCS spectrum or that uses an LBT etiquette. The PACS-UB protocol meets the FCC UPCS requirements by using a "blinking beacon" protocol. This means that the RPs 54 transmit "beacon" signals that MTs 52 use to find RPs with which they can establish a communications channel. This "blinking beacon" is preferably accomplished using the PACS-UB hyperframe and superframe structures, illustrated in FIG. 2. FIG. 2 illustrates a preferred PACS-UB hyperframe 200 which comprises 30 one second superframes 202.
This arrangement satisfies the FCC requirement that an RP 54 may only hold a acquired frequency for 30 seconds, unless it establishes communication with an MT 52. These superframes 202 are system broadcast channel superframes (SBC-SF). One type of superframe is called an access superframe. An RP broadcasts an access superframe when it is attempting to acquire a channel. FIG. 2 shows an access superframe 202' in detail. The access superframe 202' is preferably divided into four phases 204, 206, 208, 210. These phases are:
1. a first 200 ms period 204 (phase A) during which an active RP 54 transmits incoming call alerts or short messages to the MT 52; PA1 2. a second 200 ms period 206 (phase B) during which an active RP 54 transmits system and other service information; PA1 3. a 400 ms period 208 (phase C) for idle ports to select a channel on which to transmit based on signal strength; and PA1 4. a third 200 ms period 210 (phase D) during which the MTs 52 select an RP 54 having the best signal strength to which the MT has access rights (i.e., the MT is from the same system). PA1 This the interference threshold; PA1 RSSI.sub.n is the ranked RF energy (e.g., signal level) detected by a received signal strength indicator (RSSI), wherein the rank is from lowest to highest (e.g., RSSI.sub.2 is the second lowest RF energy); and PA1 kTB is a background noise level. PA1 TX is the transmission power; PA1 TX.sub.max is the maximum transmission power permitted by the FCC; PA1 TX.sub.min is a system dependent minimum transmission power; and PA1 Th is the interference threshold.
One problem with this etiquette is that a successful UPCS spectrum implementation having a large number of RPs should frame synchronize the RPs. This frame synchronization makes it likely that more than one RP will simultaneously monitor a channel for 10 ms and then acquire it, resulting in unacceptable co-channel interference. Once the synchronized RPs acquire the same channel, it is likely that they will continue to do so, rendering the ports relatively useless. To overcome this problem, it is proposed in the related application that radio ports be assigned "stagger start" times. This means that each RP begins its frequency search at a different time than nearby RPs. Thus, RPs that can detect each others transmissions do not begin their channel searches simultaneously. This avoids the adjacent port co-channel interference problem.
Another problem with the LBT etiquette is illustrated with reference to FIGS. 3 and 4. FIG. 3 is a histogram 300 showing an RP's signal level measurements of the channels in an illustrative PACS-UB protocol system in the UPCS spectrum at a period in time. PACS-UB further divides each of the eight 1.25 MHZ channels into four 300 kHz system channels, providing 32 total channels. A threshold level Th is used to determine whether or not a channel is available to an RP. If this threshold is set to a high level, such as Th.sub.1 in FIG. 3, very noisy channels will be accepted resulting in suboptimal performance. In FIG. 3, a relatively noisy channel such as channel 2 is below the threshold. Because an RP must acquire the first channel it encounters having less than the threshold level of interference, a noisy channel such as channel 2 may be acquired even though much quieter channels, such as channels 9 and 11, are available. The FCC fixes a maximum level of 30 dB above a background noise level, which background level is about -118 dBm in a PACS-UB protocol system. However, this maximum level may be increased by up to 20 dB by reducing the RP's transmission power by a corresponding amount.
If the threshold level is set very low, such as Th.sub.3 in FIG. 3, only very quiet channels will be selected. Few, if any, channels meeting this low threshold may be available, thus preventing RPs from finding an available channel. In FIG. 3, only channel 31 is below the Th.sub.3 level. If several RPs are searching for channels and these RPs "hear" approximately the same signal levels as this RP, this channel may be acquired, leaving no channels available for this and other RPs.
Thus, an intermediate threshold level, such as Th.sub.2, is desirable. In FIG. 3, at least 7 channels below this threshold level are available. Even this level, however, may not be optimal. The quietest channels are channel 9, 11, and 31. The RP will acquire the first available channel below the threshold. Here, this is channel 4. Channel 4 is about twice as noisy as channels 9, 11, and 31, and therefore less desirable. Moreover, the signal levels for the channels change over time with call arrivals and departures. At the point in time illustrated in FIG. 3, Th.sub.2 may be the preferred threshold level. At another point in time, however, a different threshold level may be better for the signal levels measured at that time.
FIG. 4 is a chart 400 showing the number of times each of the 32 channels is selected in an illustrative PACS-UB system. As noted above, the LBT protocol requires an RP to select the first channel it encounters having less than the threshold level of interference for the 10 ms period. The LBT protocol also requires an RP to begin its channel search in the lower 3 MHz of the band and search upward until an available channel is found. (For wide band systems, the search begins in the higher 3 MHz of the band.) These two requirements result in the lowest frequency channels being acquired more often than the higher frequency channels (the converse is true for wide bandwidth systems). This is illustrated in FIG. 4. The curve labeled "actual" indicates how often channels are acquired in a relatively simple UPCS system. This curve shows that the lower channels are acquired much more often than the higher channels. This results in an inefficient use of the system resources. The ideal channel utilization distribution is uniform as seen in FIG. 4 by the line labeled "ideal": each channel is acquired as often as any other, making maximum use of all available channels.
To approach this ideal distribution, the threshold level should be adjusted to an appropriate level which is neither too high nor too low for the relevant conditions of the system. The relevant conditions, however--such as RP spacing, current UPCS spectrum usage, and local propagation conditions--vary from system to system and even from time to time within the same system. Moreover, the optimal threshold value may differ between RPs in the same system, and may vary for each RP over time as the relevant conditions change. Thus, it is difficult or impossible to select a single threshold value that will provide a channel distribution approaching the ideal at all times even for a single system.
The threshold level, in some instances, is related to the RP transmission power. The FCC has limited the maximum transmission power for devices operating in the UPCS spectrum.
This maximum power level is proportional to the square root of the transmission bandwidth. In the PACS-UB protocol, this maximum transmission power is about 17 dBm. The FCC allows the interference threshold to be increased from the 30 dB maximum, provided that the transmission power be decreased a corresponding amount below the maximum. For example, in a crowded, noisy environment, an RP may require an interference threshold of 35 dB. The FCC permits the threshold to be increased to this level, provided that the transmission power is reduced by 5 dBm. Thus, in the PACS-UB example discussed here, the maximum transmission power is reduced to 12 dBm.
In some instances, it is preferable to reduce the transmission power to reduce interference between RPs. This is illustrated in FIG. 5. FIG. 5 is a diagram illustrating one floor of a building 500 having a UPCS system. The building comprises three parts, an office suite 502, having a high density of RPs 54, a factory 504, having an intermediate density of RPs 54, and a warehouse 506, having a low density of RPs 54. The RPs' transmissions create coverage areas 508 which are indicated in FIG. 5 as a circle (or part of a circle). Preferably, the RPs 54 are arranged so that the entire area is covered by the coverage areas 508. Thus, an MT 52 will be in wireless communication with an RP 54 regardless of the MTs location in the building 500. In this example, to avoid unnecessary interference with nearby RPs, the coverage areas 508 in the office 502 cover less area than those in the factory 504 or the warehouse 506 (some coverage area overlap is preferred to prevent gaps in the RP coverage). Thus, the transmission power of the office 502 RPs 54 is lower than that of the RPs 54 in the factory 504 or the warehouse 506. Also, the transmission power of the RPs 54 in an area need not be the same. For example, in the warehouse 506 in this example, RP 54' has a higher transmission power than RP 54". In this instance, if the conditions affecting the interference threshold change, the interference threshold may change adaptively, and the transmission power should be able to change accordingly.
It is an object of the present invention to provide a method for adaptive, autonomous interference threshold level setting.
It is another object of the present invention to provide a UPCS system that effectively uses all available channels.
It is yet another object of the present invention to provide a method for adaptively, autonomously setting a transmission power level.